It is known that, to enable them to be handled, optical fibres are covered by a protective sheath. In the production of optical fibre elements, such as for example, cables, the optical fibres coated with said protective sheath are placed in appropriate housings which are generally constituted by polyolefin material.
Examples of such housings are loose sleeves described in U.S. Pat. Nos. 4,143,942, 4,153,332, 4,676,590, 4,688,889, and UK-1.598.540 and grooved cores described in U.S. Pat. Nos. 4,199,224, 4,491,386, 4,668,889, 4,690,498; EP-A-280.279; UK-1.448.793, 2.021.282, 2.099.173, 2.164.471, 2.170.921, 2.172.410, 2.174.822, and FR-2.200.535.
It is also known that in optical fibre elements other structures of polyolefin material can be present.
It is further known that hydrogen exerts harmful effects on optical fibres and jeopardizes their efficiency. It has, therefore, been proposed to incorporate, in optical fibre elements, some compositions capable of absorbing hydrogen before it exerts its harmful effects on the fibres. In this specification, such compositions are, for brevity, referred to by the expression "H.sub.2 -absorbing buffers".
The applicants or their assignee, have developed H.sub.2 -absorbing buffers substantially consisting of compositions comprising at least one unsaturated organic compound of silicon and at least one hydrogenation catalyst selected from the group comprising the salts and the organic and inorganic compounds of the transition metals.
Examples of these compositions are described in U.S. Pat. No. 4,688,889 and in the U.S. patent application Ser. Nos. 07/723,441 and 07/722,085, both filed Jun. 27, 1991.
It has now been observed unexpectedly some polyolefin materials poison the hydrogenation catalyst, which is based on transition metals, of the H.sub.2 -absorbing buffers.
Studies carried out with the object of identifying the cause of such poisoning have shown that this must be looked for in some additives added to protect the polyolefin against degradation due to oxidation (anti-oxidants) and/or heat (stabilizers) and/or the proximate metals (metal deactivators), in spite of the fact that, usually, the quantity of such additives is only of the order of 2.5.times.10.sup.-2 to 5.times.10.sup.-2 parts by weight with respect to the quantity of H.sub.2 -absorbing buffer in contact with the polyolefin material.
The additives which inactivate the H.sub.2 -absorbing buffer are identified by means of a test, described in detail later, with which it is possible to measure the quantity of hydrogen absorbed by a H.sub.2 -absorbing buffer before and after it has been placed in contact with a polyolefin, protected against oxidation and/or heat degradation, for 30 days at 100.degree. C. in an air atmosphere saturated with water vapour at 150 mbar.